Field of the Invention
The invention relates to a process and an apparatus for preparation of octachlorotrisilane and polychlorosilanes from a monomeric chlorosilane, by exposing the chlorosilane to a nonthermal plasma and recycling chlorosilane that has not been converted to octachlorotrisilane and higher polychlorosilane into the plasma.
Discussion of the Background
The related art discloses processes for preparing polychlorosilanes. For instance, DE 10 2006 034 061 A1 discloses a reaction of silicon tetrachloride with hydrogen to prepare polysilanes. Because of the reaction in the presence of hydrogen, the polysilanes prepared contain hydrogen. In order to be able to keep the plant in continuous operation, tetrachlorosilane is added in excess in relation to the hydrogen. In addition, the plant disclosed has a complex structure and allows only the preparation of polysilane mixtures. An elevated molecular weight of the polysilanes can be achieved only through series connection of a plurality of reactors and high-frequency generators. After passing through each of the series-connected plasma reactors, there is an increase in the molecular weight of the polysilanes after each plasma reactor. The process disclosed is restricted to the preparation of compounds which can be converted to the gas phase without decomposition.
EP 1 264 798 A1 discloses a process for workup of by-products comprising hexachlorodisilane in the preparation of polycrystalline silicon.
U.S. Pat. No. 4,542,002 and WO 2009/143823 A2 also disclose plasmatic processes for preparation of polychlorosilanes proceeding from silicon tetrachloride and hydrogen. As a result of the preparation, hydrogen-containing polychlorosilanes are obtained. According to WO 2009/143823 A2, mixtures of hydrogen-containing high molecular weight polychlorosilanes are obtained. The silicon tetrachloride present in the polychlorosilanes has to be removed by distillation under reduced pressure in a costly and inconvenient manner prior to further use. A particular disadvantage in the prior art is the need to prepare the polychlorosilanes in the presence of gaseous hydrogen. As a result, very high safety demands are placed on the materials and the safeguarding of the plant.
Typically, octachlorotrisilane, abbreviated to “OCTS” in the context of the invention, and higher polychlorosilanes are produced by conducting a gas mixture through a series of plasma reactors. Each pass through one of the plasmas increases the mean molar mass of the gas mixture.